drm-tests: fix double buffering for null_platform_test

BUG=chromium:434924
TEST=run test on /dev/dri/card0

Change-Id: Ie66a5e9083366d156d52dd8c38f80d0a8d7229ad
Reviewed-on: https://chromium-review.googlesource.com/231392
Commit-Queue: Haixia Shi <hshi@chromium.org>
Tested-by: Haixia Shi <hshi@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: Ilja Friedel <ihf@chromium.org>
1 file changed